Since there is no vaccine to prevent visceral leishmaniasis (VL) (kala-azar), a disseminated intracellular protozoal infection, its practical management revolves around successful drug therapy. Pentavalent antimony (Sb) remains the conventional, albeit suboptimal and now less effective, treatment of choice. To formulate new therapeutic strategies in kala-azar, the two logical complementary approaches are (a) new drug development (amphotericin B (AmB), miltefosine) and (b) identification of immunologic mechanisms which can be regulated and translated into treatment. This application focuses on immuno-regulation of successful host responses to antileishmanial chemotherapy and is directed at the effects of T cells and cytokines. This area of research, immunochemotherapy, is relevant to the field with future as well as already-realized clinical impact. The overall objective is to optimize the host response to treatment by characterizing and then manipulating key immuno-regulatory mechanisms in favor of the host receiving chemotherapy. The unifying hypothesis is that, once targeted, discrete mechanisms can be stimulated or inhibited and then joined with chemotherapy to enhance initial intracellular Leishmania donovani killing in the tissues and also prevent post-treatment relapse. To accomplish the objective and test this translational research strategy, this work wilt be carried out in a clinically relevant sequential fashion and will (a) be directed at in vivo responses, (b) test hypotheses in established infection and (c) incorporate more than one chemotherapeutic agent to probe the host response to current (Sb) as well as more newly used treatments (AmB). Three related Specific Aims support the overall objective and will advance the analysis: Aim 1: Determine how lFN-gamma (Th1 cell-associated response) regulates the response to Sb and converts its action from leishmanistatic to leishmanicidal. Aim 2: Test the hypothesis that synergy with antileishmanial chemotherapy can be induced by specific, linked pharmacologic intervention designed to raise intrinsic host T cell reactivity by: (a) neutralizing the suppressive effect of endogenous interleukin 10, (b) enhancing interleukin 12 action by inhibiting cyclooxygenase-2 (COX2), and (c) triggering antileishmanial T cell costimulatory pathways, CD28:B7 and CD4OL:CD4O. And Aim 3: Characterize immunostimulation in AmB's efficacy, and in recrudescent infection following AmB therapy, pinpoint the likely cytokine-driven T cell mechanism which prevents post-treatment relapse.